A sight glass is a transparent tube through which an operator of a tank, boiler, gear box, or other machine can observe a level of liquid (such as oil or hydraulic fluid) contained within. A sight glass includes glass or a polymeric material that acts as a reflector when no liquid is present. When liquid is in contact with the glass or polymeric material, light passes through the sight glass. Accordingly, the presence of liquid in the system can be quickly verified. In other known systems, the sight glass is replaced with a sensor.